I never believed in once saved always saved until very recently. I grew up in "church" but I didn't know Jesus. I thought once saved always saved meant you could do whatever you wanted and be saved. Of course that sounds ridiculous!
John 10:26-29 says: But you do not believe, because you are not of My sheep...My sheep hear My voice, and I know them, and they follow me. And I give them eternal life, and they shall never perish, neither shall anyone snatch them out of My hand. My Father who has given them to Me is greater than all and no one is able to snatch them out of my hand.
Now, when I was not His sheep, I had no conviction of anything in my life except the heresies I was taught as a child. I didn't walk in love, I didn't act like a christian except for one day a week, I loved to watch horror movies, party at the clubs, drink, and act like the rest of the world.
From the moment I got saved, the Holy Spirit entered into my life and I honestly don't WANT to do any of those things.
I KNOW that if I mess up and sin, God is not giving up on me and taking my salvation away. I am positive that I am saved! If I sin and get into a car wreck before I get a chance to ask for forgiveness, I know I'm still going to heaven.
It seems like, since we have free choice, we could make a decision to give our salvation back, but I can't imagine ever wanting to do that. Jesus said, If they went out from us, they were not with us to begin with, that has to do with those like Judas, who appear to be christians but have their own agenda or selfish purposes for being christian.
Those who think they can lose their salvation, put your trust in Him and He will faithfully complete the good work which He has started in you!